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Topic: Suspended Projetor Screen (Read 1577 times)

Hello all and thanks for checking out my project! I hope to gain some insight on what I'm doing wrong or didn't think of yet.

I currently have a structure build that is suspended from my ceiling that is to hold a projector screen made from a sheet. The screen is between the projector and myself as this sheet is thin enough to let the light shine through with minimal light and clarity loss. I have an array of wheels and rope set up in such a way that when I pull one joint cable it raises or lowers the sheet from either side. I have the ropes set up so that one end is tied to either side, it drops down to wrap around the bottom of the sheet, then the opposing side is what actually moves and raises the sheet. this is supposed to lend hand to a bottom bar that actually rolls as it is raised, rolling the sheet around it. However, I can not yet figure out how to have a bar on the bottom that will both roll and not flex under its own weight. Here are my questions:

1. how could I better arrange the pulleys to support this weight? my current idea is shoddy at best but functional. they consist of a guide wheel for the vertical segments of rope, an angled guide wheel to either redirect the rope to the opposite side or to angle the rope into joint rope that I use to move the screen, and a final set to guide the pull cord.2. how best could I make this bar for the bottom of the screen? I have tried gray rods around white 8-way connectors but that flexed way too much as it is about 6 feet long. I have tried a much sturdier rectangular bar made from purple corner connectors, small white rods, and the tiniest green rods but that had the same problem. If it didn't shatter under its own weight, it would flex beyond what would make the screen viewable. I would like to have this bar roll as it is being raised so that the sheet rolls around it.I have considered mounting chain to the rope to force this bar to roll but there are two problems with this, I don't yet have a sturdy bar for the bottom, and the current arrangement of wheels/pulleys would have to change as the k'nex chain does not bend against its axis.

3. Is there any potential for this to be motorized? maybe with clever gear ratios? speed is not an issue. this is of much less importance than the above questions.

Thanks for your time and thought. I am at my wits end at least until some drunken epiphany happens. Also, I can upload some more pictures or videos of whatever angle and element you may need to give more informed help.

This is actually quite similar to something I made a few years ago, so hopefully I can help!

The first thing I notice is that the rope is slipping off because it is being pulled off to the side as the projector screen rises. I fixed that issue in mine by having the pulleys parallel to the screen, so they turn in the same plane. This will allow the string to wrap further around the pulley as it gets pulled diagonally by the screen. If you need the string to turn perpendicular to the screen, just use a second pulley above the first one. Basically, for each turn the string makes, make sure the pulley is oriented in the same plane.

I would also recommend using a much thinner rope. Thicker rope has a lot more friction and is generally harder to work with. You could get away with some very thin nylon string.

As for motorizing it, you could build a spool, though the amount of string that needs to be wound up would require a fairly large one. Another thing you could try is having several pulleys in a line that are geared in opposite directions, and having the rope sort of snake in-between them. With enough pulleys, it should be able to grip the rope and hold it tight. Attach a weight to the loose end so it doesn't tangle and just let it hang somewhere (maybe use a block-and-tackle on the weight so it doesn't have to hang down so far). If you have trouble getting the pulleys to grip onto the string, put a thick rubber band around the inside of the pulley to give it some more friction on the string.

That's actually really nice. I'm going to try and take your ideas for the pulleys. I have not had an issue with the rope yet though other than when it slips off of the wheels like you noticed. I'm going to blame that issue on the fact that this is the first prototype after nothing but thought therefore some measurements are off. Lol. But one thing I did notice with your screen is how the sheet saggs in the middle. I'm trying to avoid that since this not much higher than my head and thus my sheet, at it's lowest while raised, is chin high. This is mainly a problem when shutting down the room. I am typically well inebriated by this point and that may get in the way and cause a problem. Regardless, I am trying to make this workable in a lower ceiling environment as I would like to take it with me when I move in the coming months and will no longer be graced with the attic I currently have it in. Thanks for the input though. The pulleys are a feature of this project lol